A lot of power plants such as photovoltaic power plants only feed electric power in a single-phase AC power grid or in one phase of a three-phase power grid. With increasing output power of photovoltaic power plants, however, it becomes more and more important to distribute the power fed in a three-phase AC power grid evenly over all three phases. This can be done by separate single-phase inverters feeding in different phases of a three-phased power grid. However, three-phase inverters directly providing three-phase output power to be fed in the three-phase power grid are preferred.
Published US patent application US 2007/0179720 A1 (corresponding to DE 10 2006 003 904 A1) discloses a method of converting direct voltage in three-phase alternating voltage by means of a plurality of single-phase inverters.
Published German patent application DE 199 37 410 A1 discloses a three-phase photovoltaic inverter which at first inverts the output voltage of photovoltaic modules in a first high frequency intermediate alternating voltage. A transformer is used to increase the voltage of the first high frequency intermediate voltage. The resulting second high frequency intermediate voltage is rectified to have an intermediate DC voltage. This intermediate DC voltage is then inverted in a three-phase output voltage.
German utility model DE 20 2006 001 063 U1 discloses a three-phase inverter for feeding electric power generated by a photovoltaic generator. The direct voltage received from the photovoltaic power generator is first converted by a DC/DC-converter, before it is inverted in a three-phase output voltage to execute a MPP (Maximum Power Point)-tracking, i.e. to vary the output voltage of the photovoltaic generator to maximize the electric energy supplied by the photovoltaic generator.
International patent application publication WO 2007/036374 A2 describes a single-phase inverter for two DC power generators and a method of operating this inverter. The two power generators are connected together so that one power generator has a positive and one has a negative potential with regard to a reference potential of the alternating voltage output by the inverter. The inverter itself is based on a combination of two buck converters.
U.S. Pat. No. 4,445,049 discloses a DC to AC inverter which delivers a sinusoidal (sine) wave form to a load. The input to the inverter is a DC source, and a gate controllable switch is connected in series with the DC source. At least two steering switches connect the controllable switch to opposite sides of the load. The two steering switches are alternately gated at the fundamental frequency of the desired sinusoidal wave form, alternately providing current in a first direction through said load and in a second direction opposite to the first direction. The gate controllable switch is pulse width modulated to achieve a sinusoidal output current wave form.
U.S. Pat. No. 3,278,825 discloses a static inverter utilizing a modified Scott-transformer. This static inverter comprises two identical inverter channels. The DC voltage which is to be changed into a three-phase AC voltage is furnished by a direct current source, which may be an array of solar cells. The output stage of each inverter channel is typically comprised of a pair of switch means which are connected to opposite terminals of the primary winding of a center-tapped transformer. The two switch means in the output stage are alternately closed thereby causing current to flow alternately in opposite direction through the transformer primary winding and back to the source. The closing of the switch means is controlled by an oscillator which supplies a sine wave at the desired frequency to a push-pull driver stage. The driver stage in turn generates square wave control pulses which are alternately applied to the switch means in the output stage. The current which flows in alternate directions in the primary winding of the transformer in the output stage generates a voltage which is approximate a square wave. This square wave will be coupled to the secondary winding of the transformer and then to a filter stage. The filter stage attenuates the higher harmonics of the square wave signal thus producing a sine wave output. The sine wave output of the filter is fed to a two-phase to three-phase conversion device such as a Scott transformer having two partial output transformers which are connected in T configuration. The voltages which are supplied to the primary windings of the two transformers which comprise the Scott-T are maintained 90° out of phase by a phase control unit.
Rhyne, Earl; Bratton, Dave: “Scott-T connected 3-phase inverters for telecommunication applications”, PROCEEDINGS TELECOMMUNICATIONS ENERGY CONFERENCE, 19 Oct. 1986 (1986-10-19)-22 Oct. 1986 (1986-10-22), pp 461-468, XP 002521585, discuss the topology of a Scott-T connected inverter.
Badin, A. A. ET AL: “Three-phase series-buck rectifier with split DC-bus based on the Scott transformer”, POWER ELECTRONIC SPECIALIST CONFERENCE 2008, PESC 2008, IEEE, IIEE, Piscataway, N.J., USA, 15 Jun. 2008 (2008-06-15) pp 516-522, XP 031300024 present a three-phase buck rectifier based on the Scott transformer.
German patent DE 39 34 836 discloses a railroad electric equipment in which two single-phase generators supply alternating voltages to the two primary windings of a Scott-transformer. The three-phase current output by the secondary windings of the Scott-transformer is first rectified and then inverted in a three-phase current of a desired grid frequency. This current is then fed in a three-phase power grid.
There still is a need for a three-phase inverter for converting DC power provided by a photovoltaic power generator in three-phase AC power having a small number of parts but nevertheless providing both for a galvanic separation of its output side from its input side and an AC output power of high grid conformity.